Marek's herpesvirus infection results in either a productive involvement of bursal and thymic cells which undergo lysis, or in a non-productive infection in peripheral lymphocytes. Some of the latter are presumptively altered in that they are responsive to viral antigens. The objectives of our proposed studies are to (1) characterize the proteins synthesized in thymic and bursal cells and in the circulating lymphocytes in terms of their immunologic reactivity with MDV antibody or antigens. In addition (2) the lytic or protective function of thymic or bursal cells on MDV infected and control lymphoid cells will be examined. Finally, (3) the affect of turkey herpes infection preceding Marek's herpes infection on the above parameters of macromolecular and immunologic functions of the lymphoid system will be examined.